Members of the YABBY gene family are characterized by two conserved domains, a C2C2 zinc finger-like domain towards the amino terminus and a helix-loop-helix, which we have called the YABBY domain, with sequence similarity to the first two helices of the HMG box towards the carboxyl end of the protein. In addition to the conserved YABBY and zinc finger domains, YAB1, YAB2, YAB3 and YAB5 display some sequence similarity on the carboxyl side of the YABBY domain. The region between the YABBY and zinc finger domains is variable amongst family members.

The presence of these domains suggests these proteins function as transcription factors. Database searches suggest that this family may be plant-specific, as genes with a similar arrangement of zinc finger and YABBY domains do not occur in either Caenorhabditis elegans or Saccharomyces cerevisiae.

The YABBY gene are expressed in a polar manner in all lateral organs produced by apical and flower meristems. Their transcripts are detectable only in the abaxial domains of lateral organs when their primordia emerge and begin to differentiate. Loss of polar expression of these genes results in loss of polar differentiation of cell types in lateral organs, and gain-of-function alleles result in abaxialization of lateral organs. The expression of these genes is precisely correlated with abaxial cell fate in mutants in which abaxial cell fates are found ectopically, reduced or eliminated. We propose that members of this gene family act redundantly to specify abaxial identity in lateral organs produced by the apical and flower meristems in Arabidopsis.